The following idiom happens often in my code:

           if {[info exists a($x)]} {do_something $a($x)}

Problem: the above does the hash lookup twice. Argh.
A natural answer is:

           catch {do_something $a($x)}

But unfortunately, this is even slower (from 7.6 to 8.4.14) due to the
exception unwinding overhead.
Moreover, it is not strictly equivalent since it loses any exception
from [do_something].
To restore equivalence, we'd need pattern matching on the error...
even slower !

Any better idiom doing just one hash lookup ?

Another approach would be to have "array defaults": an option to
substitute a common value to all undefined slots of an array instead
of raising an exception. This one would also allow things like a blind
"incr a($x)".
I remember an old discussion on this. What's the current status ?

Yet another approach would be more general, and akin to a Bourne Shell
mechanism: "variable defaults".
The idea is to have ${foo:-bar} yield $foo or bar if foo is undefined.
The natural extension to arrays yields

            do_something ${a($x):-nope}

(here [do_something] is supposed to handle 'nope' innocuously, at
least faster than a 2nd hash lookup)

Reactions ?

I wonder, does the Tcl interp "cache" array lookups so that immediately
following identical look ups are faster? Doubtful.

My thought (all of 10 seconds):
    info value a(x) defaultValue
Could have info exists baked-in and if non-existent,
just return the supplied default otherwise return the
wanted value.

Roy